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e e . Flood control alternatives were evaluated using an updated design precipitation event of 3.46 inches (using the City of Fort Collins, revised in 2000, I-hour, 100-year estimated precipitation) for a storm with a 6-hour duration. See Appendix E for details on the hydrologic analysis associated with the current study. Figure 3 shows the hydrograph of the 100- year runoff flow produced from the model for the current study at the L- W Canal. The 100-year runoff hydro graph produced from this model shows two distinct peaks. The first (earlier) peak of slightly larger than 4,000-cfs is generated from the watershed areas downstream of the Douglas Reservoir. The second (later) peak of slightly greater than 5,000-cfs, is generated by flows originating upstream of Douglas Reservoir. DESCRIPTION OF CONCEPTUAL ALTERNATIVES Initially three conceptual alternatives: (1) Diversion to Poudre River, (2) Large "Main Stem" Storage, and (3) Storage and Diversion were identified. To avoid stressing a downstream reservoir on the L- W canal, concepts with storage and diversion alternatives require a diversion back to the Cache La Poudre River. This could be accomplished with an East Vine Diversion. Additional study improved the storage and diversion alternative by developing four options built around similar themes. Each of these options utilized the East Vine Diversion to bring the Dry Creek flows back to the Poudre River. The three distinct alternatives are: ALTERNATIVE 1 - DIVERSION TO POUDRE RIVER VIA "WILLOX LANE DIVERSION" This concept called the Willox Lane Diversion diverts the peak flow out of the watershed upstream of the confluence with the Poudre River and reduces the flood flows entering the Lower Basin from upstream. The plan view of this alternative is shown on Figure 4. From a construction cost perspective this alternative was viable. However, technical concerns regarding collecting 5,000-cfs in the wide floodplain of Dry Creek and diverting the flows into a structure under the L- W Canal as well as the railroad tracks, Willox Lane and utilities are difficult. Diverting flood flows upstream of the natural confluence with Dry Creek and the Poudre River would impact structures located in the Poudre River floodplain upstream of the confluence with Dry Creek. An additional concern was the diversion of flood flows out of the Dry Creek basin because of potential water rights concerns on the Poudre River. The Boxelder Creek Diversion alternative was rejected due to the regulations that prohibit the trans-basin diversion of flood flows. ALTERNATIVE 2 - LARGE STORAGE Under existing conditions a 100-year flood would generate about 2,200 acre-feet of runoff at the L-W Canal. If this water could be stored in a reservoir and released slowly, the flood flow generated from the Upper and Middle Basin can be reduced to a discharge that greatly reduces the cost for new facilities in the lower basin. No potential singular dam site with sufficient volume to store the entire flood could be identified in the area upstream of the L- W Canal 15